World of Warcraft

@[email protected] Gave a very thorough explanation already, so I will just add some additional bits.

World of Warcraft had a lot of "expansions" over the years, which change the game in some very basic ways and sometimes even remove old content and Blizzard doesn't keep every expansion online at the same time, there are like "phases". For example right now you can play Classic/Era (no expansion), Cataclysm (third expansion), and War Within (latest expansion, aka retail). But some time ago there were servers for TBC (first expansion) and WotLK (second expansion), which by now have transitioned to Cataclysm (third expansion), and those will progress to MoP (fourth expansion) sometime later this year.

There is something to be said for trying the content chronologically, i.e. starting with Classic/Era, but with the release model Blizzard uses progressing "historically" is hardly possible on the official servers. There are however unofficial servers which can facilitate this to a degree (using them might be considered illegal though depending on your jurisdiction, but AFAIK nobody ever got punished for playing on them, only for hosting).

I'd ignore Hardcore and Seasons of Discovery until you know a bit more what you are doing, as they are modifications to the original.

Whichever expansion you choose, you will want to look into user made UI modifications, which are referred to as "addons". E.g. for the missing quest markers on the map in Classic/Era you can find addons that add them.